This invention relates to optical fiber canisters, and, more particularly, to a technique for preventing optical fiber breakage at optical fiber winding transitions between layers.
Optical fibers are strands of glass fiber processed so that light beams transmitted therethrough are subject to total internal reflection. A large fraction of the incident intensity of light directed into the optical fiber is received at the other end of the optical fiber, even though the optical fiber may be hundreds or thousands of meters long. Optical fibers have shown great promise in communications applications, because a high density of information may be carried bidirectionally along the optical fiber and because the quality of the signal is less subject to external interferences of various types than are electrical signals carried on metallic wires. Moreover, the glass fibers are light in weight and made from a highly plentiful substance, silicon dioxide.
Glass optical fibers are typically fabricated by preparing a preform of glasses of two different optical indices of refraction, one inside the other, or a single glass composition with a coating that ensures totat internal reflection, and processing the preform to a fiber by drawing, extruding, or other process. The optical fiber is coated with a polymer layer termed a buffer to protect the glass from scratching or other damage. As an example of the dimensions, in a typical configuration the diameter of the glass optical fiber is about 125 micrometers, and the diameter of the optical fiber plus the polymer buffer is about 250 micrometers (0.010 inch).
For such a very fine optical fiber, the handling of the optical fiber to avoid damage that might reduce its light transmission properties becomes an important consideration. The optical fiber is typically wound as a layer onto a cylindrical or slightly tapered cylindrical bobbin with many turns adjacent to each other in a side-by-side fashion. After one layer is complete, another layer of the optical fiber is overlaid on top of the first layer, and so on, in a back-and-forth pattern.
At the end of each layer, a transition winding arrangement termed a "step back" is used to provide a smooth transition between the end of the layer just completed, and the beginning of the next layer about to be wound. The step-back transition reduces the chances of a snarl in the optical fiber as it is dispensed or payed out from the canister. The arrangement of the wound optical fiber is termed the "fiber pack", and the final assembly of the bobbin and the fiber pack, optionally with a housing, is termed a "canister". At a later time when the optical fiber is to be used, the optical fiber is dispensed from the canister in a direction generally parallel to the axis of the bobbin. The point at which the optical fiber is separated from the optical fiber pack during payout is termed the "peel point". This peel point moves around the circumference of the bobbin as each turn of optical fiber is dispensed, and also moves forwardly or rearwardly as each layer is dispensed.
It has been found by experience that, where the optical fiber is to be payed out from the canister in a rapid fashion, the turns of optical fiber may need to be held in place on the canister with an adhesive. The adhesive holds each turn of optical fiber in place as adjacent turns and overlying layers are initially wound onto the canister, and also as adjacent turns and overlying layers are payed out. Without the use of an adhesive, payout of the optical fibers may not be uniform and regular, leading to snarls or snags of the optical fibers that damage them or cause them to break as they are payed out.
Notwithstanding the use of the adhesive, in the transition step-back region at the end of every other layer of the fiber pack at the end toward which the optical fiber is dispensed, there is a greater likelihood of the optical fiber snapping as the winding pattern changes. In these transition regions, the optical fiber winding arrangement changes from one in which the optical fiber must be lifted over adjacent turns in order to be drawn from the bobbin (the "hard payout" orientation), to one in which the optical fiber is drawn directly from the bobbin (the "easy payout" orientation). The step-back winding eases this transition, but even then there remains a greater likelihood of failure, a phenomenon termed "aft end" or "dispensing end" breaks.
There have been attempts to vary the amount of the adhesive or its method of application in the transition step-back region in order to reduce the incidence of dispensing end breaks. These attempts have not been successful. The transition step-back region continues to have a relatively higher probability of peel point failure than most other regions of the optical fiber pack.
Accordingly, there is a need for an improved approach to preventing dispensing end breaks during the dispensing of an optical fiber from a canister. The solution must be operable over the entire temperature range specified for operation of the dispensing canister. The present invention fulfills this need, and further provides related advantages.